As demand for higher quality pipes or tubes grows in recent years, there is an increasing trend that nondestructive test standards for the pipes or tubes (hereinafter referred to as “pipes” when deemed appropriate) are becoming more stringent.
For example, a seamless pipe, which is a typical pipe, is manufactured by punching a billet with a piercer to form a hollow shell and rolling the hollow shell with a mandrel mill or the like. The seamless pipe has flaws having various tilt angles (hereinafter referred to as “tilted flaws” when deemed appropriate) with respect to the axial direction.
A tilted flaw is believed to be caused by deformation in the axial direction of a longitudinal crack originally existing on the billet in the above manufacturing process or transfer of a flaw existing on a guide face of a guide shoe for maintaining a path center of a hollow shell. Therefore, the tilt angle of the tilted flaw with respect to the axial direction of the seamless pipe changes depending on a difference in a pipe diameter of the seamless pipe or a cause for occurrence thereof. That is, there are tilted flaws with various tilt angles on the seamless pipe.
Since there is a trend of tighter service conditions of the seamless pipes from year to year, higher quality is demanded and accurate detection of the above tilted flaws is also sternly demanded.
Conventionally, various methods for detecting the tilted flaws existing on the seamless pipes have been proposed.
In Japanese Laid-Open Patent Publication No. 55-116251 (hereinafter referred to as “Patent Literature 1”), for example, a method for detecting a tilted flaw by arranging an ultrasonic probe at an appropriate position and tilt angle depending on the position and tilt angle of the tilted flaw to be detected is proposed.
However, the method described in Patent Literature 1 has a problem that extremely much time and manpower are needed because the tilt angle of the ultrasonic probe must be changed each time in accordance with the tilt angle of the tilted flaw to be detected. Also, to detect tilted flaws with various tilt angles existing on the seamless pipe in one round of flaw-detecting work, as described above, many ultrasonic probes must be provided, each of which is arranged with a different tilt angle. That is, there are problems that large equipment is required and soaring costs are entailed, in addition to complicated arrangements/settings and calibration of ultrasonic probes.
To solve the problems of the method described in the above Patent Literature 1, a flaw detecting method that applies an ultrasonic phased array probe in which a plurality of transducers (elements for transmitting/receiving ultrasonic waves) are arranged in a single row is proposed in Japanese Laid-Open Patent Publication No. 61-223553 (hereinafter referred to as “Patent Literature 2”). More specifically, ultrasonic shear waves are propagated within the pipe by aligning an arrangement direction of the transducers with the axial direction of the pipe and arranging the ultrasonic probe decentralized from an axial center of the pipe. Then, according to this method, the tilted flaws with the various tilt angles are detected by changing the tilt angle (tilt angle with respect to the axial direction of the pipe) of ultrasonic waves transmitted and received by the ultrasonic probe using electronic scanning that electrically controls transmission/reception time-shift of the ultrasonic wave by each transducer.
However, the method of Patent Literature 2 has the following problems mainly.
FIG. 1 shows a diagram illustrating an example of a relation between the tilt angle (angle formed by an extension direction of the tilted flaw and the axial direction of the pipe) of the tilted flaws and echo intensity according to an ultrasonic testing method applying an ultrasonic phased array probe, verified by an experiment conducted by the inventor of the present invention. More concretely, FIG. 1 shows echo intensity (relative intensity when the echo intensity of a tilted flaw with the tilt angle 0° is defined to be 0 dB) of each tilted flaw when, in a state where an ultrasonic phased array probe equivalent to that described in Patent Literature 2 is arranged with a constant eccentricity from the axial center of the pipe, the tilt angle of the ultrasonic wave is changed by electronic scanning in accordance with the tilt angle of each tilted flaw so that the extension direction of the tilted flaw and a propagation direction (propagation direction viewed from a normal direction of a tangential plane of the pipe including an incident point of the ultrasonic wave) of the ultrasonic wave transmitted by the ultrasonic probe are orthogonal to each other. The inventor of the present invention has found a problem that, as shown in FIG. 1, echo intensity is different depending on the tilt angle of the tilted flaw even if the tilted flaw is of the same size (0.5 mm in depth and 25 mm in length).
The inventor of the present invention has found that this problem is caused by a fact that if tilt angles of ultrasonic waves are changed through electronic scanning according to the tilt angles of the respective tilted flaws in condition where an eccentricity of the ultrasonic phased array probe is set to a constant value so that an extending direction of the tilted flaws may be orthogonal to a propagation direction of the ultrasonic waves transmitted by the ultrasonic probes, an angle (internal refraction angle) formed by a normal of the pipe at a point where the ultrasonic wave (center line of an ultrasonic wave beam) that has entered the pipe reaches an internal surface of the pipe and the ultrasonic wave (center line of the ultrasonic wave beam) and an angle (external refraction angle) formed by a normal of the pipe at a point where the ultrasonic wave (center line of the ultrasonic wave beam) that has entered the pipe reaches an external surface of the pipe and the ultrasonic wave (center line of the ultrasonic wave beam) vary with:
(1) eccentricities of the ultrasonic probes;
(2) tilt angles of the ultrasonic waves owing to electronic scanning; and
(3) a ratio of thickness (t) to outer diameter (D) of the pipes (=t/D).
As described above, the inventor of the present invention has found that the method described in Patent Literature 2 has the problem that the echo intensity is different depending on the tilt angle of the tilted flaw and there is a possibility that this problem may prevent detection of a harmful flaw or lead to over-detection of minute flaws that need not be detected.
On the other hand, Japanese Laid-Open Patent Publication No. 2005-221371 (hereinafter referred to as “Patent Literature 3”) proposes an ultrasonic testing method for using an ultrasonic phased array probe in which each of transducers has a pipe-axial tilt (which corresponds to the above-described ultrasonic wave tilt angle) of an transducer derived from a tilt of a flaw (which corresponds to the above-described tilt angle of the tilted flaw) and a flaw detection angle of refraction (=above-described external refraction angle) and a pipe-circumferential angle of incidence (which corresponds to a pipe-circumferential angle of incidence determined based on the above-described eccentricities).
According to the method disclosed in Patent Literature 3, based on a pipe-axial tilt and a pipe-circumferential angle of incidence of each of the transducers, a tilted flaw having a specific tilt angle (especially, an external surface flaw present on the external surface of the pipe) can be detected highly accurately. However, this method has a problem that detectability deteriorates for a tilted flaw having a tilt angle different from that at a time of designing the ultrasonic probes.
Further, according to the method disclosed by Patent Literature 3, detectability tends to deteriorate more for an internal surface flaw than for an external surface flaw, so that the internal surface flaw may be missed. The inventor of the present invention has found that this problem is caused by a fact that an internal refraction angle becomes too larger as compared to an external refraction angle owing to a ratio of thickness to outer diameter of the pipe and a tilt angle of the tilted flaw.
Moreover, Japanese Laid-Open Patent Publication No. 5-249091 (hereinafter referred to as “Patent Literature 4”) proposes an ultrasonic testing method for detecting a tilted flaw having a desired tilt angle in condition where an angle of incidence of an ultrasonic wave upon a pipe is kept constant (that is, an external refraction angle is kept constant) by rotating an ultrasonic probe along a side face of a cone that has a point of incidence of the ultrasonic wave upon the pipe as its vertex and a normal at this point of incidence as its central axis.
However, the method disclosed in Patent Literature 4 also has a problem that detectability deteriorates more for an internal surface flaw than for an external surface flaw because an internal refraction angle becomes too larger as compared to an external refraction angle, which is a constant value, depending on a ratio of thickness to outer diameter of a pipe and a tilt angle of a tilted flaw.
In other words, the methods disclosed in Patent Literatures 3 and 4 both have a problem of difficult detection owing to deteriorated detectability of an internal surface flaw depending on a ratio of thickness to outer diameter of a pipe and a tilt angle of a tilted flaw, because setting of flaw detection condition is based on an external refraction angle that can be derived from a propagation speed of an ultrasonic wave (ultrasonic longitudinal wave) through a coupling medium that fills a gap between an ultrasonic probe and the pipe, a propagation speed of the ultrasonic wave (ultrasonic shear wave) through the pipe, and an angle of incidence of the ultrasonic wave upon the pipe in accordance with the Snell's law.